Ever-decreasing feature sizes of integrated circuit patterns have led to ever-increasing resolution requirements for projection-optical systems used to print the patterns onto a wafer or other substrate.
To improve the resolution of a projection-optical system, it has been proposed to shorten the wavelength .lambda. of exposure light or increase the numerical aperture (NA) of the projection-optical system. In general, a projection-exposure apparatus uses a light source emitting an exposure radiation in the range from g-line light (436 nm) to i-line light (365 nm).
To realize the benefits of increased resolution, distortion of the image must be reduced. Image distortion arises mainly from distortions caused by (1) the image-forming properties of the projection-optical system, (2) warping of the wafer or substrate (which is positioned on the image side of the projection-optical system and on which a circuit pattern is to be printed), and (3) warping of the reticle (which is positioned on the object side of the projection-optical system and on which the circuit pattern is drawn).
To reduce the adverse effect of warping of a wafer, a so-called image-side telecentric optical system has conventionally been used, in which the exit pupil is positioned at infinity on the image side of the projection-optical system.
To reduce the effect of warping of a reticle, it has been proposed to use a so-called object-side telecentric optical system in which the entrance pupil of the projection-optical system is positioned at infinity on the object side of the projection-optical system. Techniques for positioning the entrance pupil of the projection-optical system relatively far from the object plane are being developed.
For efficient exposure of highly integrated circuits, increased exposure area is needed in addition to improved resolution.
In a projection-exposure apparatus that uses exposure light having a short wavelength, the lens system must employ high-quality glass materials, having high transmissivity in the ultraviolet, and being capable of suppressing fluorescent radiation or solarization due to absorption of ultraviolet rays. The number of such materials is very limited. Moreover, such materials have a low refractive index, and therefore, it is difficult to correct aberrations in the lens system using such materials.
To overcome this problem, a conventional exposure apparatus uses a wavelength-selection filter through which radiation emitted from the light source passes. The wavelength-selection filter limits the exposure radiation to a substantially narrow spectral bandwidth, so that chromatic aberration is negligible. Limitations on lens design due to chromatic aberration are thus reduced to some extent, while other aberrations are corrected.
The narrower the spectral bandwidth of the exposure radiation, however, the greater the energy loss in the exposure apparatus, resulting in longer exposure times.